The subject matter disclosed herein relates to the art of rotary wing aircraft and, more specifically to active vibration control or suppression in a rotary wing aircraft.
Rotary wing aircraft, or rotorcraft, can generate significant vibratory responses during operation. The primary source of such vibration is that generated by the main rotor system rotating at the blade passing frequency. Forces and moments are transmitted through the gearbox into the airframe, resulting in airframe vibration. One approach to counteracting such vibration involves replacing a rigid gearbox mounting strut with a compliant strut and parallel hydraulic actuator. A control computer commands the actuator such that the gearbox is selectively vibrated to produce inertial forces which minimize airframe vibrations. Although effective, this approach is inadequate in certain situations, such as a vehicle having a gearbox secured directly to the airframe, without mounting struts.
Another approach utilizes a first pair of counter-rotating eccentric masses that rotate at the frequency of the primary aircraft vibration and generate a fixed magnitude vibration force. The fixed magnitude force is then paired with a constant magnitude load from a second pair of counter-rotating masses to produce a resultant vibratory force of variable magnitude and phase. This method is heavy as it requires multiple eccentric masses powered by multiple motors and often these must be enclosed in separate housings to allow for geometric alignments that minimize unwanted moments and are thus not amenable to weight reductions. A typical approach to reduce weight in such a system would be to reduce the weight of the masses, and increasing the radius of their rotation to compensate for the reduced mass. However, since the system is circular in configuration, weight of housing components increases with radius squared, this negating the desired weight reduction. Additionally aircraft sometimes experience multiple frequencies of ambient vibration caused by forward flight load on the rotor systems. The counter-rotating eccentric mass type actuator is only suitable for generating one frequency of anti-vibration load as the load frequency is determined by the rotational speed of the eccentric masses. This is undesirable as it requires multiple such anti-vibration actuators to suppress multiple frequencies of ambient vibration.
Yet another method excites a mass-spring pair that is tuned to be nearly resonant at the desired operating frequency. In this case, linear motion of the mass is limited by material stresses of the spring. Thus, increased motion requires that larger and heavier springs be utilized.